My check engine light has been on with a P0138 High voltage code, bank 1 sensor 2. I replace the 02 sensor near the manifold (before the cat) as told to with a Bosch, but the light is still one, even after i reset the code it came back on. I checked my ground to the engine block which is good, and my 15A fuses look good to. Any other ideas as to why it getting high voltage, is the rear 02 sensor causing this and need to be re-placed? Anyone else with this problem figure it out?
The rear O2 sensor *is* the one after the cat as pointed out by X99J above, so replacing the front won't resolve this code.
The P0138 high voltage code indicates an open in the rear sensor heater circuit. It may be a bad internal heater within the rear O2 sensor, you could check the heater wires (dark green/white and black) for continuity. If the sensor heater checks out OK, you're stuck with checking the rest of the circuit from the PDC to the ground stud next to the dipstick to find the open.
Last edited by Ed Rico; 05-04-2009 at 09:37 AM..
Reason: Responded too quickly, sorry for any confusion.
What I meant was using a meter to check the circuit for continuity. But to your point, checking for melted insulation would be helpful and could yield some useful clues to finding the problem. Just be careful when checking the sensor itself to probe the 2 heater wires (colors listed above) and NOT the 2 sensor output wires. You don't want to risk damaging the O2 sensor by applying a voltage across the sensor output, and besides it won't help you solve the problem.
Rear O2 sensor on OBD-II monitors oxygen content of exhaust stream downstream of the cat. This information is used to compare to the upstream (pre-cat) O2 value in order to determine if the cat is functioning normally. The rear sensor has no direct effect on air/fuel mixture, but can have an important indirect effect since the internal resistance heaters for both sensors are wired in parallel. If a problem with the rear sensor heater knocks out the fuse in the PDC, the front sensor heater is disabled as well which can negatively affect driveability.
Front and rear O2 sensors have physically different connectors, otherwise they are the same. That's why the replacement sensors that require splicing your old connector are the same for front or rear applications.
My downstream O2 sensor wires (behind the cat) fell on the driveshaft wearing through them. I am having the same hot engine misfires/sputtering problem as described in these posts and the same code. After the sputtering began I spliced the wires together but the problem continues. After reading the Upstream and Downstream O2s run on a parallel circuit I thought I would check the fuse. Do you know where it is located? I thought that possibly the Downstream O2 short may have also shorted the other Upstream o2 sensor? What do you think?
O2 sensor heater circuit fuse is located in the Power Distributon Center under the hood. If you damaged the wiring to the rear sensor, it likely shorted the heater circuit, effectively knocking out the heater to both sensors.